The present invention is a differential transconductance amplifier which is particularly useful for driving a load at high frequencies while maintaining a low level of d.c. offset current.
Because of their relatively low distortion characteristics, differential amplifiers using Class A amplifier halves are particularly useful in the magnetic recording art. Such a differential amplifier typically comprises a pair of Class A transistor amplifier halves coupled together in a symmetrical, differential configuration. In operation, it amplifies the difference between a pair of input signals and uses the amplified difference to drive a load, typically a magnetic recording head. Such differential amplifiers have proven satisfactory in relatively low speed recording systems.
In high speed recording systems, however, these amplifiers present difficulties in the form of a d.c. offset current which have not heretofore been satisfactorily overcome. The offset current derives from operation of the Class A amplifier halves. Even when a Class A amplifier is not being driven, an idling or quiescent current will pass through any load to which it is connected. This idling current can drive a magnetic recording head to generate a magnetic field and to record a d.c. level even when no information signal is presented at the input.
In relatively low speed recording systems, the magnetic field generated by the idling current may be cancelled to a substantial extent by the use of a two-coil head and two matched Class A amplifiers. By using substantially identical coils and matched transistor amplifiers having substantially identical idling currents, the system can be designed so that the two idling currents pass through identical paths in opposite directions resulting in a low-level of d.c. offset current.
This solution, however, is expensive and difficult to achieve in high frequency recording systems. This is so because, as the frequency increases, the component parts of the amplifiers must be matched with increasing accuracy. Typically this matching problem may be solved to a limited extent by providing the amplifier with a pair of high frequency transistors each having a high gain and a high frequency response. However, such transistors are expensive and difficult to match. Moreover, they may change their relative characteristics with the passage of time or even with variations in temperature. For this reason, such circuits have been successfully used only with relatively low frequency signals.